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United States Patent |
5,049,202
|
Willis
,   et al.
|
September 17, 1991
|
Method of enhancing the ductility of aluminum-zinc alloy coating on
steel strip
Abstract
A continuous hot-dip coating line, whereby an aluminum-zinc alloy coating,
comprising from 25% to 75% aluminum by weight, is applied to a steel
strip, includes an induction furnace whereby the coated strip is heated to
a treatment temperature within the range of from 165.degree. C. to
275.degree. C., preferably about 200.degree. C. The strip emerging from
the furnace is coiled directly at that treatment temperature into close
wound coils having a mass of at least 2 tons, and the coils are allowed to
cool in still air at the ambient indoor temperature pertaining, at a rate
of no more than 40 centigrade degrees per hour, for all but a
predetermined number (typically less than four, usually only one) of the
outer turns of the coil which are scrapped, to enhance the ductility of
the coating on the remainder of the coil. If the treated coated strip is
subsequently painted, the paint is cured at a peak metal temperature of
less than 240.degree. C.
Inventors:
|
Willis; David J. (Albion Park Rail, AU);
Salon; Michael (Keiraville, AU)
|
Assignee:
|
John Lysaght (Australia) Limited (Sydney, AU)
|
Appl. No.:
|
510264 |
Filed:
|
April 19, 1990 |
Foreign Application Priority Data
| Apr 24, 1989[AU] | PJ3871 |
| Aug 24, 1989[AU] | PJ5953 |
Current U.S. Class: |
148/531; 148/533; 428/653; 428/659 |
Intern'l Class: |
C21D 001/00; B32B 015/10 |
Field of Search: |
428/653,659
148/11.5 Q,13,127,437,441
427/407.1,436
|
References Cited
U.S. Patent Documents
3297499 | Jan., 1967 | Mayhew | 148/156.
|
4287008 | Sep., 1981 | Torok et al. | 148/127.
|
4287009 | Sep., 1981 | Allegra et al. | 148/127.
|
4350539 | Sep., 1982 | Torok et al. | 428/653.
|
4722871 | Feb., 1988 | Radtke | 428/653.
|
Foreign Patent Documents |
537941 | Sep., 1981 | AU.
| |
0028821 | May., 1981 | EP.
| |
55-18562 | Feb., 1980 | JP.
| |
Primary Examiner: Dean; R.
Assistant Examiner: Koehler; Robert R.
Attorney, Agent or Firm: Armstrong, Nikaido, Marmelstein, Kubovcik & Murray
Claims
We claim:
1. A method of enhancing the ductility of an aluminium-zinc alloy coating,
comprising from 25% to 75% aluminium by weight, on a steel substrate,
comprising the steps of bringing the coated substrate to a treatment
temperature within the range of from 165.degree. C. to 275.degree. C. and
cooling the coated substrate from the treatment temperature to below
121.degree. C. at a rate not exceeding 40 Centigrade degrees per hour.
2. A method according to claim 1, wherein said substrate is a strip and
said cooling step is effected by coiling sufficient of the strip at the
treatment temperature to form a close wound coil having a shape and size
such that it may be allowed to cool naturally in still air at the ambient
temperature pertaining without all but a predetermined number of the outer
layers of the coil exceeding the cooling rate of 40 centigrade degrees per
hour, allowing it so to cool, and scrapping the predetermined number of
outer layers.
3. A method according to claim 2 wherein said predetermined number is less
than four.
4. A method according to claim 2 wherein said predetermined number is one.
5. A method according to claim 2 wherein said coil has a mass of at least 2
tonnes.
6. A method according to claim 2 wherein said step of bringing the coated
strip to said treatment temperature is effected by passing the strip
through a furnace included in a continuous coating line, by which the
coating is applied to the strip, upstream of a coiler at the end of that
line.
7. A method according to claim 2 wherein said step of bringing the coated
strip to said treatment temperature is effected by operating a continuous
coating line, by which the coating is applied to the strip, in a manner
ensuring the strip passes to a coiler at the end of the line at that
treatment temperature.
8. A method according to claim 1 comprising the further steps of applying a
paint to the treated coated substrate and heat curing said paint, wherein
said heat curing is effected at a peak metal temperature of less than
240.degree. C.
9. A method according to claim 2 comprising the further steps of applying a
paint to the treated coated strip and heat curing said paint, wherein said
heat curing is effected at a peak metal temperature of less than
240.degree. C.
10. A method according to any one of the preceding claims wherein said
treatment temperature is substantially 200.degree. C.
11. A coated steel substrate having an aluminium-zinc alloy coating,
comprising from 25% to 75% of aluminium by weight, when treated by a
method according to claim 1.
Description
BACKGROUND OF THE INVENTION
This invention relates to the manufacture of steel sheets coated with
aluminium-zinc alloy having an aluminium content within the range of from
25% to 75% by weight. Typically, such manufacture is effected in large
scale plants by continuous processes which produce coils of stock strip
material for subsequent fabrication into finished products.
Limitations exist on the range of articles that may be fabricated
satisfactorily from such stock material unless the alloy coating is
sufficiently ductile to enable sharp bends or folds to be made in the
coated sheet without damage to the coating. If the coating is not
sufficiently ductile, small cracks may be created in it when the sheet is
subjected to high strain fabrication, such as, for example, being folded
on itself or bent over a die having a thickness of the order of the
thickness of the sheet. Even non-ductile coatings usually remain adherent
to the steel substrate, but if the coating has been painted prior to
forming, such severe bending may also cause the paint to develop minute
cracks. If the cracks in the paint coincide with the cracks in the
coating, over a period of time a discoloration may occur at the cracks in
the paint.
Thus it is important to ensure that the aluminium-zinc alloy coating has
sufficient ductility at the time of fabrication to tolerate high strain
fabrication without damage.
The conventional continuous coating process ensures that when the strip
emerges hot from the coating station, with the still liquid coating on it,
it is rapidly cooled, at a rate of at least 11 centigrade degrees per
second, to solidify the coating before the strip reaches the first met
roll downstream of the coating station, namely the so-called turn-around
roll. This rapid cooling produces a fine grained, dendritic structure in
the coating. That structure is essential if the coating is to have the
requisite corrosion resistance. Thereafter, subsequent processing along
the line generally continues the cooling at a relatively rapid rate to
room temperature and may include a final quench, thereby preventing any
substantial change in the crystal structure or grain size. That subsequent
processing certainly excludes any reheating, at least until the heat
curing of a paint coat, if such is applied to the strip.
Unfortunately, the ductility of the fine grained coating, as produced by a
correctly operated coating line, is below that needed to enable
satisfactory high strain fabrication, as discussed above, to be effected.
The situation is somewhat improved if, as a part of the line process, the
alloy coated strip is painted and the paint is heat cured. The heating
needed to cure the paint softens the alloy coating and enhances its
ductility, however the higher ductility so produced is transitory and
disappears on ageing at room temperature, so that, ideally, conventional
painted strip should be fabricated within a few weeks of its production.
Such a time constraint on the use of the finished strip is of course very
undesirable.
SUMMARY OF THE INVENTION
An object of the present invention is to alleviate the above-described
position.
The invention achieves that object by providing a heat treatment for an
aluminium-zinc alloy coating within the above mentioned composition range
on a steel substrate, whereby the ductility of the coating is enhanced. If
the treatment is given to unpainted material, it increases the ductility
itself and its permanence. If it is given to material which has been
painted and heat cured, it increases the permanence of the ductility
induced by the heat curing. If it is given to material prior to it being
painted and cured it is essential that the maximum temperature of the heat
cure be limited to 240.degree. C. if the beneficial effects of the heat
treatment are to be retained.
The invention consists in a method of enhancing the ductility of an
aluminium-zinc alloy coating, comprising from 25% to 75% aluminium by
weight, on a steel substrate, comprising the steps of bringing the coated
substrate to a treatment temperature within the range of from 165.degree.
C. to 275.degree. C., preferably 200.degree. C. or close thereto, and
cooling the coated strip from the treatment temperature to below
121.degree. C. at a rate not exceeding 40 Centigrade degrees per hour.
If the treatment is hastened by using a greater cooling rate the increased
ductility remains transitory and there is no real benefit if, after
treatment, a few weeks elapse before fabrication occurs, which in practice
may often be the case.
The invention may be effected by a batch annealing operation, in which a
large quantity of the freshly coated product is heated in a furnace to the
treatment temperature, and then allowed to cool with the furnace. Such a
batch operation is effective from a technical point of view, but is
unattractive commercially because of the cost of the equipment needed and
the time taken for the operation, which may be several days. That is to
say, such batch treatment implies what may be undesirably long lead times
between the placing of individual orders on the manufacturer of the stock
product and the fulfillment of those orders.
Therefore, in preferred embodiments of the invention applicable to
continuous coating of steel strip, the method is effected by coiling
sufficient of the strip at the treatment temperature to form a close wound
coil of a size and shape such that it may be allowed to cool naturally,
that is to say by exposure to still air at room temperature, without the
cooling rate exceeding the stipulated forty Centigrade degrees per hour,
at least for the bulk of the coil. We say "at least for the bulk of the
coil", because at least the outermost turn of the coil may cool at a
higher rate because of limitations on the rate of heat flow to it from the
interior of the coil and may have to be scrapped.
The actual rate of cooling at any instant is proportional to the
temperature difference between the coil's surface and its surroundings at
that instant and its surface area, and is inversely proportional to its
mass, which is proportional to its volume. It follows, for a given
temperature difference, that the rate of cooling is dependent on both the
coil's axial length and its inner and outer diameters. It is also
dependent on the thickness of the strip, because, for given inner and
outer diameters, this determines the number of turn to turn interfaces,
which affects the rate at which heat may flow to the surface of the strip,
and thus the surface temperature.
At any particular work site, limiting values for such parameters as the
strip width, strip thickness and inner coil diameter are usually well
established by the equipment available and the type of product customarily
produced. Therefore, by utilising conservative values for the minimum
likely ambient temperature, those parameters and the number of outer turns
to be scrapped, one can establish for the site, by trial and error, a
minimum mass at which a coil, apart from the outer turn or turns to be
scrapped, will not cool too rapidly. If an actual coil of that mass is
made having other parameters within the possible ranges the coil will cool
more slowly. This would increase the production time but would have no
deleterious effect on the value or stability of the ductility of the
coating.
BRIEF DESCRIPTION OF THE DRAWINGS
By way of example, an embodiment of the above-described invention is
illustrated by the accompanying drawings and described further with
reference to them.
FIG. 1 is a diagrammatic representation of a typical hot-dip alloy coating
line modified to carry out the invention.
FIGS. 2A to 6B are graphs showing comparative test results as between sheet
samples treated within and without treatment limits according to the
invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
For preference the method of the invention is carried out as the final
steps of a continuous hot dip or similar coating process. In some
instances the existing coating line may be operated, without additional
equipment, in a manner which causes the coated strip to emerge at a
treatment temperature suitable for it to be coiled at that temperature and
put aside for natural cooling as aforesaid. For example, steps in the
normal operation which have a cooling effect may be omitted. However, more
frequently, existing lines would be modified to perform the method of the
invention by the inclusion of strip heating means at or near the end of
the line, to enable the requisite treatment temperature to be attained.
The choice of the continuous process or line to be modified or operated in
accordance with the invention would be made having regard to the
circumstances of each work site. If the plant is set up to produce painted
or otherwise overcoated stock material in a single pass, then the hot
coils would be taken from the end of the line and the strip heating means,
if present, would preferably be immediately upstream of the coiler. More
usually, in existing plants, the alloy coating line and the overcoating
line are separate installations, in which event the hot coiling, with or
without dedicated strip heating means, may be effected at the end of
either line.
However, if the heat treatment of the invention is effected before a paint
cure, it is essential to ensure that, during the curing, the strip
temperature never exceeds 240.degree. C., or the enhanced stability of the
coating's ductility will be deleteriously affected.
The coating line illustrated by FIG. 1 is a dual purpose line in that it
may produce conventional aluminium-zinc alloy coated product, which is
quite suitable for many applications, as well as the heat treated,
enhanced ductility product of the invention, which is suitable for
subsequent high strain fabrication.
The line comprises a sequence of treatment stations, namely a hot dip,
alloy coating bath 1, a controlled cooling station 2, a temper rolling
station 3, a tension leveller 4, an alloy coating passivating station 5
and an accumulator 6. A bare steel strip 8, which would have been
appropriately cleaned and otherwise treated to render it able to accept an
alloy coating by conventional upstream stations (not shown), may be
traversed through the component stations 1 to 6 to a coiler 7, to produce
close wound coils of conventional product in the usual way needing no
further description. When the line is so operating, the treatment
stations, in particular the cooling station 2 and accumulator 6, normally
ensure that the coated strip passes to the coiler 7 at a temperature well
below 100.degree. C., usually less than 50.degree. C., for example close
to room temperature.
On the other hand, when the line is operating in accordance with the
invention, a strip heating furnace 9 is energised to ensure that the strip
passes to the coiler 7 at a treatment temperature within the aforesaid
range, preferably at or a little above 200.degree. C. In this instance the
passivating station 5 may be rendered non-functional if desired.
The strip heating furnace may be of any appropriate type, but for
preference it is an induction furnace because of the precision with which
such furnaces may be controlled and the speed of their response.
The furnace 9 is immediately upstream of the coiler 7 as is preferred, but
it could be elsewhere in the line if need be, provided insulatory
enclosures or the like are furnished to reduce heat loss from the reheated
strip.
In any event, the coiler 7 is operated to produce coils of the
predetermined minimum mass for the site, to enable them to be put aside
for natural cooling without exceeding a cooling rate of forty centigrade
degrees per hour, at least for the great bulk of the coil.
In trials leading to the present invention it was found that the second
outermost turn of an unpainted coil wound at a treatment temperature of
substantially 200.degree. C. and having a mass of 1.5 tonnes, an axial
length (strip width) of 1200 mm., an outer diameter of 679 mm., an inner
diameter of 508 mm. and a strip thickness of 1.7 mm. cooled, when the coil
was exposed to still air having an ambient temperature of 30.degree. C.,
at a maximum rate of 38 Centigrade degrees per hour, that is to say near
to the maximum allowable value.
Under otherwise identical circumstances a coil having a strip width of 650
mm. (and therefore an outer diameter of 795 mm. to attain the 1.5 tonne
mass) cooled at a maximum rate of 22 centigrade degrees per hour.
At the work site in question, on the central east coast of Australia, a
minimum expected room temperature would be about 10.degree. C., and the
test results were extended, by computer modelling, to find, rather
surprisingly, that the ambient temperature had only a small effect on the
cooling rate. Thus an assumed 0.degree. C. ambient, would only have
affected the cooling rates by about three centigrade degrees per hour.
Therefore, for the site in question, a minimum coil mass of, say, 2 tonnes,
on the assumption that the two outermost turns of the coil may be
scrapped, would be a safe criterion to adopt in any situation falling
within the limits of the invention. Indeed, as the strip parameters for
large continuous galvanizing plants do not differ greatly throughout the
world, the 2 tonne limit may be taken, as of today, as being generally
applicable. If a larger mass is adopted, or if the individual coils are
stacked for cooling, the maximum cooling rate would be reduced below the
stipulated forty centigrade degrees per hour limit, and although this
would increase the production time, it would not be detrimental to the
coating.
In other embodiments, in which a subsequent painting or polymer coating
line includes temper rolling and tension levelling facilities, the temper
rolling means 3, leveller 4 and passivating station 5 of the alloy coating
line may be by-passed or rendered inoperative and the cooling station 2
controlled so that coated product leaves it at a temperature sufficiently
in excess of 200.degree. C. to ensure, notwithstanding natural heat losses
occurring during the strip's transport to the coiler 7, it is coiled at
the treatment temperature of about 200.degree. C. To simplify this, the
strip pass length between the exit from the cooler 2, through the
by-passed or inoperative treatment stations, through the exit accumulator
6 and to the coiler 7 is preferably designed to be as short as possible.
Turning now to the remaining figures, it will be seen that FIG. 2 shows
graphical representations of three different parameters of crack severity
relative to the heat treatment temperature. The results were obtained by
heating coated samples to the several treatment temperatures, allowing the
samples to cool slowly within the furnace, and determining the indicated
crack parameters by microscopic examination after the cold samples had
been subjected to a standardised high strain bend. The three sets of
graphs clearly demonstrate the correlation of the improvement in ductility
with the treatment temperature range of from 165.degree. C. to 275.degree.
C., and in particular with the preferred temperature of 200.degree. C.
FIG. 3 shows the effect of the cooling rate following heating to
200.degree. C. on the stability of the increased ductility as indicated by
the crack severity, when measured, on the one hand, within a few hours of
the heat treatment and, on the other, after three months ageing at room
temperature. The figure demonstrates that the ductility after ageing
approaches the initial figures only for cooling rates below 0.67
centigrade degrees per minute, that is 40 degrees per hour.
FIG. 4 is similar to FIG. 3 but relates to samples which were subjected to
a simulated paint cure stoving following the 200.degree. C. heat treatment
and before bending or ageing.
FIG. 5 shows the crack severity in samples as seen immediately following
painting and after ageing for three months at room temperature
respectively. It demonstrates that samples that have been given the heat
treatment of the invention before painting, must be protected from peak
metal temperatures above 240.degree. C. during the paint cure if the
ductility improvement is to be long lasting.
FIG. 6 shows two graphs, one relating to samples tested shortly after
treatment and the other to samples after three months ageing. The slow
cooling of the samples was interrupted at various temperatures below the
treatment temperature of 200.degree. C. down to 63.degree. C. The samples
were given a simulated paint line stoving cycle, with a peak metal
temperature of 230.degree. C. The results demonstrate that the slow
cooling must be continued to 120.degree. C. or below if the increased
ductility is to be long lasting.
As indicated above the invention is applicable to coatings of
aluminium-zinc alloy comprising 25-75% by weight aluminium and the
remainder essentially zinc. It is also applicable to such alloys
optionally including small quantities of impurities and/or small
percentages of elements such as silicon, cerium and magnesium, known to
those skilled in the art to be used as additives in aluminium-zinc coating
compositions.
The claims defining the invention are as follows:
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